Hydrogen storage tank

ABSTRACT

The invention relates to a tank of the type that comprises a container ( 4 ) for housing hydrogen and metallic hydride placed inside the container. According to one part of the invention, the tank includes at least one solid body ( 6 ) made from a compacted material containing metallic hydride and a matrix. The invention is suitable, for example, for tanks for internal combustion engines or for fuel cells, in particular for motor vehicles, as well as for any stationary or mobile application involving hydrogen.

The present invention relates to a storage tank for hydrogen in the formof a metallic hydride, of the type comprising a container for housinghydrogen, and metallic hydride placed inside the container.

A tank of the aforesaid type may be used for supplying hydrogen to afuel cell or a heat engine.

WO 2007/011476A2 describes a hydrogen storage tank comprising a tubularcontainer inside which cells are arranged, each cell being composed of aplurality of small, sector-shaped receptacles, each receptaclecontaining metallic hydride powder.

It is an aim of the invention to provide a storage tank for hydrogen inthe form of metallic hydride which permits the storage of a large volumeof hydrogen, with satisfactory charging and discharging speeds.

To this end, the invention proposes a hydrogen storage tank of theaforesaid type, characterized in that it comprises at least one solidbody formed of a compacted material comprising metallic hydride and amatrix.

According to other embodiments, the tank comprises one or more of thefollowing features, taken singly or in all the technically possiblecombinations:

-   -   the matrix is formed of expanded graphite, preferably of        expanded natural graphite;    -   the metallic hydride is a hydride of magnesium or of magnesium        alloy;    -   the tank comprises a plurality of solid bodies stacked inside        the container in a stacking direction;    -   the or each solid body is in the shape of a pellet and is held        inside the container in such a way as to create an annular space        between the lateral inner surface of the container and the or        each solid body;    -   the tank comprises a heat exchanger having at least one duct for        a heat exchange fluid, extending inside the container;    -   the duct extends through solid bodies;    -   the tank comprises metal plates threaded onto the duct        alternately with the solid bodies;    -   the tank comprises annular spacers threaded onto the duct        alternately with the metal plates, the or each solid body being        threaded onto a spacer;    -   the duct comprises a feed pipe and a discharge pipe for a heat        exchange fluid which are substantially coaxial;    -   the duct comprises an outer tube and an inner tube extending        inside the outer tube, the inner tube defining one of the feed        pipe and the discharge pipe for the heat exchange fluid, and the        outer tube defining with the inner tube the other of the feed        pipe and the discharge pipe;    -   the feed pipe communicates with the discharge pipe via openings        distributed along the inner tube;    -   the discharge pipe is annular and surrounds the feed pipe;    -   the tank comprises heating elements for the solid bodies;    -   each heating element extends through several solid bodies; and    -   the heating elements are electrical resistances;    -   the solid bodies are held spaced apart from one another along        the stacking axis, with a gas circulation space between them;    -   the or each solid body comprises between 15% and 25% by mass of        expanded graphite, in particular around 20% by mass of expanded        graphite; and    -   the or each solid body comprises between 1 and 10% by mass of        expanded graphite, in particular between 5% and 10% by mass of        expanded graphite.

The invention and its advantages will become clearer from the followingdescription, provided solely by way of example and with reference to theappended drawings, in which:

FIG. 1 is a schematic view in longitudinal section of a hydrogen tankcomprising pellets of compacted composite material;

FIG. 2 is an enlarged view of the area II of FIG. 1;

FIG. 3 is a graph illustrating the density and the porosity of pelletsof compacted composite material, in dependence on the compactionpressure;

FIG. 4 is a schematic view of a bench for measuring thermal conductivityaccording to the divided bar principle;

FIG. 5 is a graph illustrating thermal conductivity measurements carriedout on pellets of compacted composite material;

FIG. 6 is a graph illustrating the equilibrium curve between magnesium(Mg) and magnesium hydride (MgH₂) in dependence on the temperature andthe pressure;

FIGS. 7 to 10 are graphs illustrating comparative tests of hydrogencharging of tanks for hydrogen storage in the form of metallic hydride;and

FIG. 11 is a view analogous to that of FIG. 2, illustrating a hydrogentank according to another embodiment.

FIG. 1 illustrates a tank 2 according to the invention for hydrogenstorage in the form of metallic hydride.

The tank 2 comprises a container 4 for hydrogen, defining an internalvolume 5, and a plurality of pellets 6 for hydrogen storage in the formof metallic hydride, which are arranged inside the container 4.

The container 4 is tubular. It comprises a tube 8 with longitudinal axisL, closed at one longitudinal end by a fixed cap 10, welded onto thetube 8, and at the opposite longitudinal end by a removable cap 12. Thetube 8 has a circular transverse section.

The removable cap 12 is for example screwed onto a fixing ring 14 weldedto the end of the tube 8 and extending the tube 8 radially towards theoutside. The ring 14 is equipped with openings 16 for the passage offixing members, such as bolts 18. The container 4 comprises a pluralityof bolts 18 distributed about the axis L.

The tank 2 comprises a hydrogen circulation pipe 20 passing through theremovable cap 12 in a leaktight manner. It enables the volume 5 to beconnected to a hydrogen supply source, or to a hydrogen consumptionunit, such as a fuel cell or a heat engine.

Each pellet 6 is in the form of a solid body made of a compactedcomposite material comprising magnesium hydride and a matrix formed ofexpanded graphite, preferably of expanded natural graphite (ENG). Thecomposite material and its method of preparation will be described inmore detail hereinafter.

Each pellet 6 has a peripheral contour corresponding substantially tothe cross-section of the tube 8, circular in this case. The pellets 6are aligned and stacked along the axis L within the tank 4.

Each pellet 6 has an outside diameter substantially smaller than theinside diameter of the tube 8. The result is that the tank 2 has anannular space 22 created between the pellets 6 and the lateral innersurface 23 of the tube 8, and extending along the axis L over the lengthof the stack of pellets 6.

The tank 2 comprises a heat exchanger 24 extending inside the container4, for heating and/or cooling the pellets 6 by heat exchange with a heatexchange fluid circulating within the heat exchanger 24.

The heat exchanger 24 comprises a duct 25 for heat exchange fluidextending axially in the centre of the container 4, along the axis L.

The duct 25 comprises an outer tube 26 and an inner tube 28 which arecoaxial. The inner tube 28 defines a central pipe 30 for channelling theheat exchange fluid. The outer tube 26 surrounds the inner tube 28 andwith the latter defines an annular pipe 31 for channelling the heatexchange fluid surrounding the central pipe 30.

The wall of the inner tube 28 comprises openings 32 (FIG. 2) distributedalong the inner tube 28 for the circulation of the heat exchange fluidbetween the annular pipe 31 and the central pipe 30.

The duct 25 comprises at one end a plug 33 (FIG. 1) for closing theouter tube 26 and the inner tube 28.

The opposite end of the duct 25 passes through the removable cap 12 in asealed manner, and is equipped with an end-piece 34 for connecting thecentral pipe 30 and the annular pipe 31 to a circuit (not shown) forheat exchange fluid.

The central pipe 30 and the annular pipe 31 are connected to the circuitin such a way that the central pipe 30 is a feed pipe and the annularpipe 31 is a discharge pipe. As a variant, the direction of circulationof the heat exchange fluid is reversed.

The pellets 6 are pierced at their centre. They are threaded onto theouter tube 26, being stacked along the axis L. Thus, the duct 25 holdsthe pellets 6 inside the container 4.

The plug 33 comprises a threaded pin 36. The tank 2 comprises a supportwasher 37 and a clamping screw 38 screwed onto the pin 36 to hold thepellets 6 along the axis L on the duct 25.

As shown in FIG. 2, the heat exchanger 24 comprises plates 40 insertedalternately between the pellets 6.

Each plate 40 is in the shape of a disc having an outer contoursubstantially identical to that of the pellets 6, and pierced at itscentre. Each plate 40 is threaded onto the outer tube 26.

Each plate 40 is in contact by its inner edge with the outer tube 26,and in contact by its opposite surfaces with the pellets 6.

The heat exchanger 24 comprises annular spacers 42 inserted between theplates 40 to maintain their spacing. Each pellet 6 is threaded onto aspacer 42 which is itself threaded onto the outer tube 26.

The plates 40 are intended to improve the heat exchanges between theouter tube 26 and the pellets 6 by conducting the heat between the outertube 26 and the periphery of the pellets 6. They are for examplemetallic, preferably made of copper.

The plates 40 and the duct 25 form a supporting framework for thepellets 6. The framework comprises a plurality of annular gaps, each gapbeing defined between two plates 40, and opening out radially towardsthe outside in the space 22.

The tank 2 comprises heating elements 44 extending through the pellets6.

Each heating element 44 is in the form of a metal rod extending throughthe pellets 6. Each heating element 44 passes through the removable cap12 in a sealed manner for its connection to an electrical supply circuit(not shown) for the production of heat inside the container 4 by heatdissipation by the Joule effect.

In a method of manufacture of the tank 2, a plurality of pellets 6 areproduced, the duct 25 and the heating elements 44 are fixed onto theremovable cap 12, the pellets 6, the spacers 42 and the plates 40 arethreaded onto the outer tube 26 of the duct 25, the whole is insertedinside the tube 8, then the removable cap 12 is fixed onto the ring 16.

The tank 2 thus obtained initially contains hydrogen in the form ofmagnesium hydride.

As indicated previously, each pellet 6 is in the form of a solid bodyformed of a compacted composite material comprising magnesium hydrideand a matrix (or “skeleton”) formed of expanded graphite, preferably ofexpanded natural graphite (ENG).

The material is qualified as “composite” owing to the combined use of ametallic hydride and a matrix, here made of ENG.

The composite material is intended to store hydrogen by absorption inthe form of magnesium hydride and to release hydrogen by desorption. Thedesorption of hydrogen results in the appearance of non-hydrogenatedmagnesium in the composite material. In the continuation of thedescription, for reasons of simplicity, “magnesium hydride” designatesmagnesium hydride and any fraction of non-hydrogenated magnesium ofcomposite material.

Within the framework of the invention, “compacted” designates a materialin which the density is significantly higher than that of the respectivedivided raw materials of which it is composed, in this case magnesiumhydride and ENG. The density of the composite material is 100% greater,and may be 400% greater, than that of the divided raw materials.

According to a method for preparing the pellets 6, the compositematerial is obtained by compaction of a mixture of magnesium hydridepowder (MgH₂) and of particles of ENG.

The magnesium hydride powder preferably has a grain size of between 1and 10 μm.

The mixture of powders is produced in a conventional manner, for examplein a mixer, at ambient temperature and at atmospheric pressure.

The compaction of the mixture of powders is carried out preferably byuniaxial compression, for example in a pelleting machine.

Preferably, mixing and compaction are carried out under a controlledatmosphere, in particular in order to avoid oxidation of the magnesiumhydride powder, which is pyrophoric.

The pressure exerted during compaction is selected in particularaccording to the porosity desired in the composite material. By way ofexample, a pressure of the order of 1 t/cm² (10⁸ Pa) has proved suitablefor obtaining pellets having a porosity of the order of 0.3.

The graph of FIG. 3 illustrates the density and the porosity of pelletsof compacted composite material obtained according to the invention, fordifferent compaction pressures.

Compaction increases the volume density of metallic hydride, andtherefore the hydrogen storage volume capacity. Compaction alsoincreases the thermal conductivity by reducing the voids within thematerial.

ENG is a form of graphite modified by chemical and thermal treatments.ENG is a good conductor of heat and consequently improves the thermalconductivity of the composite material. Its presence and its structurefacilitate the cohesion of the composite material. The result is thatthe composite material has a very high mechanical strength, permittingmachining of the pellets, and facilitating their handling for loadingthem into the tank.

In addition, and surprisingly, unlike a metallic hydride powder, thecomposite material is not pyrophoric, thereby making it safer to handleand facilitating in particular the charging of the tanks.

Advantageously, the magnesium hydride powder used for the mixtureincludes less than 10% by weight, preferably less than 5% by weight, ofnon-hydrogenated magnesium. The more perfectly the magnesium hydride ishydrogenated, the more stable the powder will be with respect to air.

Preferably, before the mixing step, the magnesium hydride powder isactivated, in order to have more favourable kinetics of absorption anddesorption of hydrogen. This activation step is carried out for exampleby co-grinding of the magnesium hydride with a transition metal or analloy of transition metals, or a transition metal oxide, introduced inproportions of between 1 and 10% atomic relative to the mixture.

The term “transition metal” as used here relates to chemical elementshaving in the atomic state a partially filled sub-layer d and which format least one ion with a sub-layer d and partially filled. Thoseparticularly referred to are the transition metals V, Nb, Ti, Cr and Mn.

In a particularly preferred embodiment, the magnesium hydride powder isactivated according to the teaching of the French patent applicationfiled under No. FR 06 51478 and published under No. FR 2 900 401 or thatof the corresponding international application published under No. WO2007/125253A1, by co-grinding with an alloy of centred cubic structurecomprising titanium, vanadium and either chromium or manganese,introduced in proportions of between 1 and 10% atomic relative to themixture.

The particles of ENG are advantageously in the form of elongatevermicules, with a diameter of the order of 500 μm and a length of a fewmillimetres.

As a result of the effect of uniaxial compaction, the vermicules areoriented substantially perpendicularly to the compression axis. Thisimparts to the composite material a strongly anisotropic thermalbehaviour, and facilitates the conduction of heat perpendicularly to thecompression axis.

Advantageously, the pellets 6 are obtained by uniaxial compaction alongtheir axis. The result is that the vermicules are orientedperpendicularly to the axis of each pellet 6, and the radial thermalconductivity of the pellets 6 is improved.

The thermal conductivity of the composite material depends on theproportion of ENG in the composite material.

Measurements were carried out on pellets prepared with differentcontents of ENG. The measurements were carried out on a conventionalcontinuous duty measuring bench based on the principle of the dividedbar, as illustrated in FIG. 4.

According to this principle, a sample 50 is positioned between twostandard parts 52 a, 52 b and disposed between two layers 53 of thermalinsulation, the whole being in contact on either side with a hot plate54 and a cold plate 55. Thermocouples 56 are inserted into the standardsand the sample in order to take the temperature at different placesdistributed between the plates 54, 55.

Samples were cut out from the pellets, along the axis of the pellet andperpendicularly to the axis, in order to measure the axial and radialthermal conductivity of each pellet.

Three compositions of composite material comprising 0%, 5% and 10% byweight of ENG were tested.

The graph of FIG. 6 illustrates the measurements of axial conductivity(dashed lines) and radial conductivity (solid lines), according to theproportion by weight of ENG. The average temperature of the samplesduring the measurements was of the order of 30° C.

The thermal conductivity is substantially proportional to the proportionof ENG. The radial thermal conductivity increases more rapidly with theENG content than the axial thermal conductivity.

The capacity by mass of hydrogen absorption of the composite materialdepends on the proportion of magnesium in the composite material, ENGabsorbing a priori no hydrogen.

The proportion of ENG is not particularly limited. It is selectedaccording to a compromise between the thermal conductivity and thecapacity by mass of hydrogen absorption.

A low content of ENG, of the order of from 1 to 10% by weight relativeto the final composition, already makes it possible to increase thethermal conductivity significantly. Therefore, the composite materialpreferably comprises from 5 to 10% by weight of ENG.

The kinetics of absorption and desorption of hydrogen are notsignificantly affected by the shaping of the material.

It is assumed that the material contains very few or no compoundsresulting from a chemical reaction between the activated magnesiumhydride and the ENG.

The composite material is easy to manufacture. It uses available rawmaterials, is inexpensive and does not require sophisticated equipmentfor its production and its shaping into pellets.

In operation, the hydrogen is discharged from the tank 2 by desorptionof the hydrogen from the composite material in suitable pressure andtemperature conditions, and the hydrogen is charged into the tank 2 byabsorption of hydrogen by the magnesium of the composite material insuitable pressure and temperature conditions.

The desorption of hydrogen from the composite material leads to theformation of metallic magnesium, available for subsequent absorption ofhydrogen.

The graph of FIG. 6 illustrates the curve of equilibrium betweenmagnesium (Mg) and magnesium hydride (MgH₂) according to the temperatureand the pressure.

The desorption of the hydrogen by the magnesium hydride is endothermic,and is interrupted spontaneously in the absence of the application ofheat. The absorption of hydrogen by magnesium is exothermic, and it isadvisable to evacuate the heat in order to charge the hydrogen in areasonable period of time, preventing the reaction from beinginterrupted spontaneously.

In order to charge the tank 2 with hydrogen or to extract hydrogen fromit, the hydrogen pressure within the container 4 and the temperature ofthe pellets 6 are maintained within suitable ranges.

The temperature of the pellets 6 is adjusted by means of the heatingelements 44 and the heat exchanger 24. The heating elements 44 are usedprincipally for applying heat to the pellets. The heat exchanger 24 isused principally for evacuating heat, by circulating within the heatexchanger 24 a fluid at a low temperature. The heat exchanger 24 mayalso be used for applying heat by circulating within the heat exchanger24 a fluid at a raised temperature.

During charging and discharging, the hydrogen emerges from or penetratesinto the pellets 6 through porosities in the composite material.

The exchange surface area between the hydrogen and the pellets 6 islarge, owing to the fact that it is located at the large diameterperiphery of the pellets 6. In a variant, in order to increase thisexchange surface area, there is axial play between each pellet 6 andeach plate 40, so that the gaseous exchanges take place equally via theopposed surfaces of the pellets 6.

Comparative tests, the results of which are shown in FIGS. 7 to 10, werecarried out in order to illustrate the improvement in performanceobtained with a tank according to the invention.

A first hydrogen charging test was carried out on a tank such as that inFIGS. 1 and 2, comprising a container of 270 cm³, but devoid of heatexchanger and heating elements, filled with 110 g of magnesium hydridepowder. The tank was equipped with a flow meter and with temperatureprobes.

A second test was carried out with the same tank, this time filled withpellets of 7 cm outside diameter, each pellet comprising 5% by mass ofENG, and 95% of magnesium hydride powders, the whole of the pelletsrepresenting a total weight of 250 g and still devoid of heat exchangerand internal heating elements.

The two tests were carried out after complete dehydrogenation of themagnesium hydride contained in the tank, by arranging the tank in anoven in order to heat the tank to an initial temperature of 300° C.,then placing the tank under hydrogen pressure of 0.8 MPa.

The graphs of FIGS. 7 and 8 illustrate the volume of hydrogen charged,the temperature at the centre and the temperature at the periphery ofthe tank for the first test (FIG. 7) and the second test (FIG. 8).

The volume of hydrogen absorbed by the pellets is greater than thatabsorbed by the powder: 170 NL against 65 NL (Normo Litres).

This is due to the fact that the compaction of the pellets increases thevolume density of metallic hydride, and therefore the hydrogen storagevolume capacity.

The hydrogen charging speed is greater with the pellets, and thisalthough the mass of magnesium and therefore the amount of heat to beevacuated is multiplied by a factor of more than 2.

Comparison of the temperatures taken at the centre and at the peripheryof the container shows that the temperature is more homogeneous in thepellets than in the powder.

These results show that the compacted composite material makes itpossible to improve the thermal conductivity and the hydrogen storagevolume capacity.

FIGS. 9 and 10 illustrate the third and fourth tests carried out bymeans of the same tank, equipped this time with the heat exchanger andheating elements. The tank thus conforms to that of FIGS. 1 and 2.

The third and fourth tests were carried out starting from differentconditions. The initial conditions of the third test (FIG. 9) are:temperature 300° C., and pressure 1 MPa. The initial conditions of thefourth test (FIG. 10) are temperature 220° C., and pressure 1.6 MPa.

Charging is more rapid starting from 220° C.

Compared with the second test, it is noted in addition that charging ismuch more rapid with the heat exchanger. This is due to the fact thatthe reaction of hydrogenation of magnesium is very exothermic, and thatin the absence of effective evacuation of the heat, the states ofequilibrium are reached very rapidly. The reaction kinetics become veryslow and lead to a very long charging time. The reaction may even beinterrupted spontaneously in the parts of the tank which are least wellcooled.

Comparison of the first test with the third and fourth tests shows agreater absorption volume in the second test. This is due solely to thefact that the amount of magnesium hydride was greater in the second testowing to the absence of a heat exchanger which occupies part of thevolume of the tank.

The improved thermal conductivity of the pellets 6 facilitates theadjustment of the temperature of the pellets 6, thereby enabling thepellets to be maintained in temperature conditions favourable to rapidcharging and discharging of hydrogen.

In addition, the presence of the heat exchanger facilitates themaintenance of the pellets in conditions that are more favourable forthe charging of the tank with hydrogen.

Moreover, the arrangement of the pellets 6 in the tank 2 permitseffective wetting. The annular space 22 between the pellets 6 permitsgood circulation and good distribution of the hydrogen, with a largecontact surface area with the pellets 6. The annular clearance which isproduced between the pellets 6 and the lateral inner surface of thecontainer 4 permits expansion of the pellets during successive chargingand discharging of the tank.

The high mechanical strength of the pellets 6 is not altered by therepetition of cycles of charging and discharging of the tank 2.

FIG. 11 is a view analogous to that of FIG. 2, illustrating a tankaccording to another embodiment. The reference numbers for elementssimilar to those of the first embodiment have been retained.

The tank 2 of the second embodiment differs from that of the firstembodiment in that the heat exchanger 24 is devoid of metal platesinserted between the pellets 6.

The pellets 6 are threaded directly onto the outer tube 26 of the duct25 of the heat exchanger 24, being kept spaced apart from one another bytubular spacers 60 inserted between the pellets 6 and enabling anannular space 62 to be kept free for the circulation of the gasesbetween each pair of adjacent pellets 6.

The pellets 6 have an ENG content of about 20% by weight relative to thefinal composition.

The increase in the ENG content to about 20% makes it possible toincrease the thermal conductivity of the pellets 6, and thus tocompensate for the absence of metal heat exchange plates insertedbetween the pellets 6.

The omission of the metal plates makes it possible to limit the cost ofthe tank and its weight. This also makes it possible to avoid reactionof the metal forming the plates with the magnesium. The copper of platesmade of copper could in time react with the magnesium to form a MgCucompound.

The tank of FIG. 11 has a gravimetric hydrogen capacity, i.e. the massof hydrogen storable in the tank compared with the weight of the pellets6, increased with respect to that of the tank of FIG. 2, i.e. the massof hydrogen storable in the tank compared with the weight of the pellets6 added to that of the metal plates.

Measurements carried out on the tanks of FIGS. 2 and 11 showed agravimetric capacity of 2.9% by mass of hydrogen for the tank of FIG. 2,against a gravimetric capacity of 4.88% by mass of hydrogen for the tankof FIG. 11.

Maintaining a spacing between the pellets 6 enables a large exchangesurface area to be obtained between the pellets 6 and the hydrogen,which makes it possible to obtain satisfactory hydrogencharging/discharging speeds.

Measurements carried out in a similar manner to that of the fourth testshowed that a tank configuration without metal plates but with aproportion of ENG increased to about 20% by weight in the pellets 6makes it possible to obtain hydrogen charging/discharging speeds and anamount of hydrogen stored of the same order as for the tank of the firstembodiment, with metal plates, and with a proportion of ENG of about 5%by weight in the pellets 6.

The proportion of ENG of the pellets 6 is between 15% and 25%, inparticular about 20%, to permit significant hydrogen storage whilefacilitating the thermal exchanges.

In a variant, the pellets 6 are not spaced apart, or are replaced by asingle one-piece solid body.

The invention applies in particular to tanks for internal combustionengines or for fuel cells, in particular in motor vehicles, and moregenerally for any stationary or mobile application.

1. A tank for storing hydrogen in the form of metallic hydride, of thetype comprising a container (4) for housing hydrogen, and metallichydride disposed inside the container, characterized in that itcomprises at least one solid body (6) formed of a compacted materialcomprising metallic hydride and a matrix.
 2. A tank according to claim1, wherein the matrix is formed of expanded graphite, preferablyexpanded natural graphite.
 3. A tank according to claim 1, wherein themetallic hydride is a hydride of magnesium or of magnesium alloy.
 4. Atank according to claim 2, comprising a plurality of solid bodies (6)stacked inside the container (4) in a stacking direction (L).
 5. A tankaccording to claim 1, wherein the or each solid body (6) is in the formof a pellet.
 6. A tank according to claim 1, wherein the or each solidbody (6) is held inside the container (40) in such a way as to create anannular space (22) between the lateral inner surface (23) of thecontainer (4) and the or each solid body (6).
 7. A tank according toclaim 2, comprising a heat exchanger (24) comprising at least one duct(25) for a heat exchange fluid, extending inside the container (4).
 8. Atank according to claim 7, wherein the duct (25) extends through solidbodies (6).
 9. A tank according to claim 8, comprising metal plates (40)threaded onto the duct (25) alternately with the solid bodies (6).
 10. Atank according to claim 9, comprising annular spacers (42) threaded ontothe duct (25) alternately with the metal plates (40), the or each solidbody (6) being threaded onto a spacer (42).
 11. A tank according toclaim 7, wherein the duct (25) comprises a feed pipe (30) and adischarge pipe (31) for a heat exchange fluid which are substantiallycoaxial.
 12. A tank according to claim 11, wherein the duct (25)comprises an outer tube (26) and an inner tube (28) extending inside theouter tube, the inner tube (28) defining one of the feed pipe and thedischarge pipe for the heat exchange fluid, and the outer tube (26)defining with the inner tube the other of the feed pipe and thedischarge pipe.
 13. A tank according to claim 12, wherein the feed pipecommunicates with the discharge pipe via openings (32) distributed alongthe inner tube (28).
 14. A tank according to claim 12, wherein thedischarge pipe (31) is annular and surrounds the feed pipe (30).
 15. Atank according to claim 1, comprising heating elements (44) for thesolid bodies (6).
 16. A tank according to claim 15, wherein each heatingelement (44) extends through several solid bodies (6).
 17. A tankaccording to claim 15, wherein the heating elements (44) are electricalresistances.
 18. A tank according to claim 4, wherein the solid bodies(6) are held spaced apart from one another along the stacking axis, withspaces (62) for gas circulation between them.
 19. A tank according toclaim 18, wherein the or each solid body comprises between 15% and 25%by mass of expanded graphite, in particular about 20% by mass ofexpanded graphite.
 20. A tank according to claim 9 wherein the or eachsolid body comprises between 1 and 10% by mass of expanded graphite, inparticular between 5% and 10% by mass of expanded graphite.